Steam turbine power plant and method of operating same

ABSTRACT

A method of operating a steam turbine facility which includes a turbine with at least a high pressure or a medium pressure cylinder and a steam reheater connected to the turbine and a valve-control bypass line connected across the reheater. During nominal and steady state operating modes of the facility, cold, steam leaving the turbine&#39;s high or medium pressure cylinder is split into two streams controlled by the valves. One stream amounting to 90%-50% of the steam leaving the turbine is fed to the bypass line while the other stream amounting to 10%-50% thereof is fed to the reheater for heating to a temperature of 650°-850° C. at a pressure not exceeding 0.1-1 Mpa. The streams of reheated and bypass steam are then merged and then mixed to form a single combined stream which is fed back to the turbine. The method enhances the cost effectiveness and reliability of the facility by reducing to zero the steam moisture once the expansion process in the turbine is complete.

FIELD OF THE INVENTION

The invention relates to the steam power field and, more particularly,it can be used in reheat steam turbine power plants as well as innon-reheat steam turbine power plants during their retrofitting byintroducing an additional steam reheating.

DESCRIPTION OF PRIOR ART

The method of operating a steam turbine power plant is known to thoseskilled in the art and according to which steam, expanded in the firstturbine blade stages, is supplied from the turbine to the steamreheaters where additional heat is added to it. After the steam isreheated, the steam is returned to the turbine to expand through thenext blade stages (see, for example, V. Ya. Rizhkin "Steam electricalpower stations", M-L, Energy, 1967, p. 30-36, 50-54).

The steam turbine power plant is known to those skilled in the art toimplement the above-mentioned known method. The steam turbine powerplant generally comprises a boiler with a primary superheater, a steamturbine consisting of a high pressure cylinder (HPC), an intermediatepressure cylinder (IPC) and a low pressure cylinder (LPC), boilerreheaters connected to the turbine with steam pipes, an electricalgenerator, a condenser, water pumps and a feed water heating systemconnected to the boiler with piping (see, for example, V. Ya. Rizhkin"Steam electrical power stations", M-L Energy, 1967, p. 30, FIG. 3-1; p.35, FIG. 3-3; p 67, FIGS. 6-4, 6-5).

When operating system turbine power plant, the said method and the steamturbine power plant allow the diagram steam moisture content to bereduced at the end of the expansion process in the turbine from 12% . .. 15% to 7% . . . 8% in comparison with a non-reheat steam turbine powerplant thereby increasing the efficiency and reliability of the steamturbine power plant.

However, the terminal moisture existing there reduces the efficiency andreliability of these steam turbine power plants and in addition to this,the said steam turbine power plants have a reduced flexibility understartup-shutdown conditions due to a considerable difference between thesteam temperature entering from the reheaters and the turbine metaltemperature.

The nearest known method to the invention is the method of operation ofa steam turbine power plant according to which, during startupconditions up to the rated load and before supplying steam from the HPCto the IPC, the cold steam leaving the HPC is divided into two flows:one flow is supplied to the reheater for reheating, while the other flowis directed to a bypass line bypassing the reheater. Both steam flowsare controlled by a control element provided in the bypass line. Then,the bypassed cold steam is mixed with the reheated steam into one flowand this mixture is supplied to the turbine. After the rated load hasbeen achieved and under the next steady state operation conditions, thecold steam through the bypass line is stopped by closing the controlelement and the steam turbine power plant is operated with one steamflow from the HPC to the IPC that was exposed to reheating in thereheater (see "Typical Instruction for starting the turbine from variousthermal conditions and for shutting down 300 MW Units with LMZ's 300 MWturbine K-300240 working in a Unit principle", M. Sojuztechenergo, 1980,p. 12").

The nearest known plant to the invention is a steam turbine power plantfor implementing the above said known method consisting of a boiler withthe primary superheater, a steam turbine comprising a high pressurecylinder (HPC), an intermediate pressure cylinder (PC), a low pressurecylinder (LPC), a reheater connected to the turbine with steam piping, abypass line with a control element bypassing the reheater, an electricalgenerator, a condenser, water pumps and a feed water heating systemconnected to the boiler with piping (see USSR Patent No. 134030, class F01 D 17/00 published in 1987).

The said known method and steam turbine power plant expedite the heatingand cooling processes in the turbine during startups and shutdowns byreducing the temperature difference between the steam entering from thereheater and the turbine metal temperature.

However, at the rated and other stationary steady-state operatingconditions, that method and that steam turbine power plant have areduced efficiency because considerable moisture appears downstream fromthe last stage of the turbine which is not removed. That residualmoisture reduces turbine efficiency and causes erosion wear in themoving turbine blades.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to increase theefficiency and reliability of a steam turbine power plant by eliminatingthe erosion wear of the moving turbine and diminishing to zero thediagram steam moisture content at the end of the steam expansion processin the turbine.

Briefly, in accordance with my method of operating a steam turbine powerplant, the plant is provided with a cold steam supply from the turbineto the reheater and a reheated steam supply to the turbine. The coldsteam leaving the turbine is divided into two flows when operating thesteam turbine power plant at rated and other steady-state operatingconditions. These flows are distributed by means of control elements anddirected as follows: one flow in amounts of 90% . . . 50% of the steamflow leaving the turbine is directed to the bypass line of the reheater,the second flow in amounts of 10% . . . 50% is supplied to the reheater.In the reheater, the steam is heated to a temperature of 650° C. . . .850° C. at a maximum pressure of 0.1 Mpa . . . 1 Mpa. Then the reheatedsteam flow and the cold bypassed steam flow are combined to one flow andmixed in a steam-mixing vessel before being returned to the turbine.

The steam turbine power plant for implementing my method comprises aboiler with a primary superheater, a steam turbine consisting of a highpressure cylinder, an intermediate pressure cylinder, a low pressurecylinder, a reheater connected to the turbine with steam piping, abypass line with a control element, a valve bypassing the reheater, anelectrical generator, a condenser, water pumps and a feed water heatingsystem connected to the boiler with piping. It also includes at leastwith one steam-mixing-vessel mounted in the steam line from the reheaterand in particular between the low pressure turbine and the bypass lineacross the reheater.

BRIEF DESCRIPTION OF THE DRAWING

In The Drawing:

FIG. 1 shows a schematic diagram of a steam turbine power plant forpracticing my method with a two-cylinder steam turbine;

FIG. 2 shows a schematic diagram of a steam turbine power plant forcarrying out my method with a three-cylinder steam turbine.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, my steam turbine power plant comprises aboiler 1 with a primary superheater, consisting of a high pressurecylinder (HPC) 2, an intermediate pressure cylinder (IPC) 3, (FIG. 2only), a low pressure cylinder (LPC) 4, a reheater 5 connected to theturbine with piping 6,7, a bypass line 8 provided with a controlelement, namely a valve 9, for bypassing the reheater, an electricalgenerator 10, a condenser 11, water pumps 12, a feed water heatingsystem 13 connected to the boiler 1 with piping 14, a steam mixingvessel 15 disposed in steam piping 7 between LPC 4 and the bypass line8, an additional control element, namely a valve 16, disposed in piping6 between the bypass line 8 and the reheater 5.

The steam turbine power plant could also be provided with an additionalreheater 5 and a bypass line 8 with the control valve 9 disposed betweenHPC 2 and IPC 3 (which is not shown in FIG. 2).

The invention is realized in the following way. After startup procedureswhen the electrical generator 10 has achieved a rated load and duringsubsequent steady-state operation conditions of the steam turbine powerplant, the cold steam leaving the turbine is divided in two flows at thedividing point A in the piping 6. These two flows are distributed bymeans of the control elements, namely valves 9, 16, and directed asfollows: one steam flow in amounts of 90% . . . 50% of the steam flowleaving the turbine is directed to the bypass line 8, the other flow inamounts of 10% . . . 50% is supplied to the reheater 5 where the steamis heated to a temperature of 650° C. . . . 850° C. at a maximumpressure of 0.1 Mpa . . . 1 Mpa. Then the reheated steam flow and thecold bypassed steam are combined into one flow at the point B in thepiping 7 and mixed in the steam-mixing vessel 15 before being returnedto the turbine.

In comparison with the prior art methods described at the outset, themethod of operating a steam turbine power plant in accordance with thisinvention increases the efficiency and reliability of the steam turbinepower plant by increasing the turbine efficiency and eliminating theerosion wear of the moving turbine blades. More particularly, the steamsupply in the range of 10%-50% of the steam flow leaving the turbinethrough the reheater 5 provides a possibility of reducing the pressurein the reheater 5 (at the same steam flow volume and velocity) up to 0.1Mpa . . . 1 Mpa by reducing the weight flow by 2 . . . 10 times that.This in turn makes it possible to reduce the stress level with the samestrength margin in the components of the reheater 5 and to increase thetemperature up to 650° C. . . . 850° C. A special mixing vessel 15, forexample of an aerodynamic or mechanical type, provides an equaltemperature steam mixture at a short section of the piping 7 andexcludes the possibility of temperature disturbances in the turbinecomponents and parts. The additional control valve 16 provides a normaloperation of the steam turbine power plant when the valve 9 fails in anemergency that increases the operational reliability of the steamturbine power plant.

When practicing the invention in a 200 MW steam turbine power planthaving initial steam conditions of 13 Mpa, 540° C. and an additionalreheater at 2.1 Mpa, 540° C. (FIG. 2), a zero moisture content isprovided at the end of the steam expansion process in the turbine. Thisoccurs when the weight of steam flow through the reheater 5 is equal to25% of the steam flow leaving the turbine at a pressure of 0.12 Mpa andtemperature of 650° C. When using the invention in a 100 MW steamturbine power plant having initial steam conditions of 9 Mpa, 535° C.(FIG. 1), a zero moisture content is provided at a steam flow throughthe reheater 5 in an amount of 35% of the steam flow leaving the turbinewith a pressure of 0.2 Mpa and a temperature of 650° C.

As a result, the efficiencies of the FIGS. 1 and 2 steam turbine powerplants are increased by 1.5% and 4% respectively and there is no need toimplement other design or technological measures and to carry outrepairs associated with erosion wear and failure of the turbine blades.

From an analysis of turbine operating conditions it was found that steambecomes moist downstream from the turbine last stage when the steam flowthrough the reheater 5 is less than 10% and steam becomes superheated bymore than 60° C. when the steam flow is more than 50% of that which isinadmissible from the operational conditions of the steam turbine powerplant.

What is claimed is:
 1. A method of operating a steam turbine power plantof the type including a turbine, a reheater connected to the turbine, asupply of cold steam from the turbine and a supply of reheated steam tothe turbine, during rated and steady-state operating conditions of theplant, said method comprising the steps ofdividing the supply of coldsteam from the turbine into first and second flow streams; directing thefirst flow stream amounting to 90%-50% of the total supply from theturbine to the bypass line; directing the second flow stream amountingto 10%-50% of the total supply from the turbine to the reheater forheating to a temperature of 650°-850° C. at a maximum pressure of0.1-1.0 Mpa, and combining the first and second flow streams from thebypass line and the reheater into a single combined stream whichconstitutes said supply of reheated steam to the turbine.
 2. The methoddefined in claim 1 and including the additional step of providing atleast one steam mixing device between the turbine and the bypass line tomix the streams from the bypass line and the reheater before they aresupplied to the turbine.
 3. A method of operating a steam power plant ofthe type including a turbine with high, medium and low pressurecylinders, a reheater connected across the medium and low pressurecylinders and a bypass line connected across the reheater, during ratedand steady-state operating conditions, said method comprising the stepsofdividing the supply of cold steam from the turbine into first andsecond flow streams; directing the first stream to the bypass line;directing the second stream to the reheater; regulating the first andsecond flow streams so that from 90%-50% of the supply of cold steamfrom the turbine flows through the bypass line and from 10%-50% of thesupply of cold steam from the turbine flows through the reheater;controlling the reheater so that the second stream is heated to atemperature of 650° to 850° C. at a pressure not exceeding 0.1-1.0 Mpa;combining and mixing flow streams from the bypass line and reheater intoa combined stream, and conducting the combined stream to the lowpressure cylinder as said supply of reheated steam to the turbine.